Mr, Patients Air Pumpf Gazometer, i^c. 95 



drilled as at h^ fig. 2nd, and which when it is in the posi- 

 tion! it now is, opens a communication between the receiver 

 and the barrel ; — we will now now suppose the piston close 

 to the bottom as in the figure, if the cock be turned as 

 soon as the aperture at h passes the hole leading from the 

 receiver K, all communication between the barrel, and it 

 is cut off, the air therefore in the barrel is driven out 

 through the valve at I. There is upon the piston about the 

 eighth of an inch of mercury represented by the dots, but 

 before this can come in contact with the top of the barrel 

 it must have expelled every particle of air, and if the pis- 

 ton ascends any higher this mercury is itself expelled 

 through the same aperture and is retained by the rim as 

 represented by the dots, and the valve I, floats upon it. If 

 the piston be now made to descend, the mercury runs 

 back until the weight of the valve brings it in contact 

 with the aperture; it then is effectually closed, with still a 

 quantity of mercury round it : the piston continues to de- 

 scend until it gets nearl-y to the bottom, when the tube h 

 coinciding with the hole in the tube aaa lets in the air 

 from the receiver K. Fig. 2nd shows the manner of con- 

 necting the tube aaa wiih the collar F, and also the diago- 

 nal hole L through the piston rod used instead of a valve. 

 The lower part of the barrel with its valves H and G, acts as 

 a common pump, and the air is admitted though the cock M. 

 Another instrument I enclose for your inspection is a 

 simple method of making the common gazometer hold 

 nearly double the usual quantity of gas, and which may be 

 used instead of Pepys' ingenious gas holder. ABCD is 

 the common gazometer, E is the outer cylinder with its 

 frame AB. F is the inner cylinder, which instead of being 

 soldered tight to the bottom as usual is screwed down by 

 three or four angular pieces as in fig. 4th ; at G is a valve 

 opening inwards, and which is shut by a pretty strong 

 spring, (an inch and a half of strong watch main spring is 

 sufficient,) to the valve is connected the brass wires aaa, 

 and at H the wire goes through a collar, and has a thumb 

 screw upon it. To use it, press down the cylinder E as 

 usual, turn the thumb screw H, which opens the valve G, 

 pour in water and the air all escapes through the tube K, 

 let the gas enter at D and the cylinder E rises, when it is 

 full close the aperture at D and press down the cylin- 



